The apparatuses and methods for measuring micro-geomorphy of the sea bottom at present may be summarized as follows: For example, in a paper entitled “Principal Components Array Processing for Swath Mapping” by P. H. Kreautner and J. S. Bird in Proceedings of the IEEE Oceans' 97 Conference, October, 1997, and a paper entitled “Beyond Interferometry, Resolving Multiple Angles-of-Arrival in Swath Bathymetric Imaging” by P. H. Kreautner and J. S. Bird in Proceedings of the IEEE Oceans' 99 Conference, September, 1999, a sonar array comprised of six equal-spaced parallel linear arrays is presented. The parallel linear arrays are made of piezoelectric ceramic arranged at regular space and connected to the terminals of the pre-amplifiers and power amplifiers. A watertight house containing all of these parts is placed underwater and connected to an overwater electronic subsystem through a cable. It operates at a frequency of 300 kHz, and transmits simple pulse signal. A “principal component array processing” method is used in signal processing. The following experiments are performed:
(A) Measurements with a man-made target are performed in a pool. The man-made target is a good acoustic target made of orthogonal copper pipes. Said target is correctly detected by the sonar system, but the results of the measurements in connection to the wall of the pool are rather poor.
(B) Experiments are performed in a small lake. The depth of the lake is 2-30 meters. The sonar array is mounted on a common tripod, which is positioned on the lake bottom in front of a small dock. The electronic apparatus is placed on the shore. The results of the experiments show that the direct arrival echoes from the lake bottom are detected, but none of the multipath signals generated from the multipath effect is deleted automatically within the action range, these multipath signals are retained in the map.
(C) The normal sidescan map is obtained by rotating the sonar array positioned in the same small lake with a stepping motor. Only the intensity of the back scattering signal can be displayed on a normal sidescan map. Though the tendency of the variation of the landform of the lake bottom may be deduced from said map, the depth of water can not be obtained.
(D) The sonar array is moved back and forth on one side of a little ship. When the attitude correction is not adopted and the apparatuses for positioning and navigating is lacking, a three-dimensional acoustic image, i.e., the three-dimensional tendency of the variation of the landform of the lake bottom, is obtained. Said apparatus is unable to give the precision of depth measurement and contour map. The depth data in the vicinity of the nadir of the sonar are lacking for the experiments performed in the pool and lake with all of the prior art apparatuses and technologies.
There are two main defects in a prior art bathymetric sidescan sonar technology. First, the depth data in the vicinity of the nadir of the sonar can not be measured correctly, even said data may be obtained, the error of measurement is rather large. Second, the echoes arriving concurrently from different directions can not be differentiated, so that the apparatus can not work normally when the multipath effect exists in the underwater acoustic channel, or the landform is complex. Therefore, the precision of measurement, action range, operation efficiency, and adaptability of the sonar are limited seriously. A “principal component array processing” method is used by P. H. Kreautner and J. S. Bird to perform signal processing. Said method is capable of differentiating the echoes arriving concurrently from different directions basically, but it fails to select automatically the wanted echoes from the lake bottom. Besides, both the precision of measurement and the contour map can not be obtained.
In order to overcome the main defect of supplying a poor precision of measurement in the vicinity of the nadir of a prior art bathymetric sidescan sonar, three methods have been adopted ever. The first method is decreasing the distance between the survey lines, it always leads to a decrease of the action range of one side, thus makes the ranges of two successive measurements being overlapped each other and the efficiency is decreased notably. The second method is adding subbottom profiler at the center. The resolution is low, because the beam width of the conic beam of said instrument is about 40°. In addition, because of its lower frequency, the penetration depth of the conic beam into the sea bottom of the water is rather large, this leads to a lower precision of depth measurement. The third measure is adding a minitype multi-beam sounding system, thus the complexity of the equipment and the price of the equipment are increased.
Second, the prior art method of signal processing of the bathymetric sidescan sonar is the differential phase estimation method. Because the echoes arriving concurrently from different directions can not be differentiated when using said method, the action range and the adaptability of the prior art bathymetric sidescan sonar are limited seriously. In addition, when working on a complex landform, a plurality of echoes arriving concurrently from different directions may be generated, thus the precision of measurement of a prior art bathymetric sidescan sonar will decrease significantly.